Since active vitamin D is chemically unstable to heat, light, oxygen and moisture, it is very important to provide it in the form of a stabilized preparation.
Conventionally, as methods for stabilizing such active vitamin D in the solid state, have been proposed the inclusion of a heat-labile vitamin D.sub.3 active compound (hereinafter it may be referred to briefly as "active vitamin D.sub.3 ") with cyclodextrin [Published unexamined patent application 128417/1976], the complex formation with cholesterols [Published examined patent application 51948/1987], the formation of the outer layer containing an active vitamin D.sub.3 and an excipient which is soluble in an organic solvent, for example, polyvinylpyrrolidone or hydroxypropyl cellulose on the inner layer comprising an excipient which is insoluble in an organic solvent, for example, lactose or crystalline cellulose [U.S. Pat. No. 4,729,895] or the like.
However, preparations provided by conventional methods of stabilization are directed to active vitamin D.sub.3, and, besides, these preparations are not stable enough under conditions of high temperature and humidity, which is counted, among other factors, as a drawback.
Furthermore, these methods for stabilization of active vitamin D.sub.3 are not applicable to stabilization of a heat-labile vitamin D.sub.2 active compound (hereinafter it may be referred to briefly as "active vitamin D.sub.2 ") as they are.
Under such circumstances as above, the present inventors examined the influence of pH on the stability of active vitamin D.sub.2. More specifically, to a Britton-Robinson broad range buffer solution (90 ml) was added an ethanol solution of 1.alpha.-hydroxy vitamin D.sub.2 (1.alpha.--OH--D.sub.2) (1 ml, containing 32 .mu.g of 1.alpha.--OH--D.sub.2), and the mixture was stored at 25 C. After six hours and 22 hours, the respective residual rates of 1.alpha.--OH--D.sub.2 were examined. The results are shown in Table 1.
TABLE 1 ______________________________________ Conditions Period pH3 pH5 pH7 pH9 pH10 ______________________________________ Residual 6 hr 48.0 52.2 67.5 87.0 88.6 content (%) of 22 hr 20.2 30.4 54.2 85.9 88.3 1.alpha.-OH-D.sub.2 ______________________________________ Method of Determining Residual Content Apparatus: Shimadzu LC6AD Column: YMCPACK A202 S5 120A C8 Volume of Sample: 150 .mu.l (Conc. of Sample 147.2 ng/ml) Column Pressure: 25 kg/cm.sup.2 Column Temperature: 40.degree. C. Mobile Phase: acetonitrile:water = 75:25 (V/V) Flow Rate: 1 ml/min Detector: UV detector 265 nm
As is clear from Table 1, it was revealed that 1.alpha.--OH--D.sub.2 was stabilized at pH 9 and higher. Based on this finding, the present inventors have further made intensive research and accomplished the present invention.